1. Field of the Invention
The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
2. Description of the Background Art
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers which are designed so that the level of damping force can be properly controlled in accordance with the road surface conditions, vehicle running conditions, etc., with a view to improving the ride quality and the steering stability.
This type of hydraulic shock absorber has heretofore been arranged as disclosed, for example, in Japanese Utility Model Application Public Disclosure (KOKAI) No. 58-70533. That is, the conventional hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein, and a piston which has a piston rod connected thereto is slidably fitted in the cylinder to define therein two cylinder chambers. The two cylinder chambers are communicated with each other by a main hydraulic fluid passage and a bypass passage. The main hydraulic fluid passage is provided with a damping force generating mechanism (including orifices, disk valves, etc.) for generating a relatively large damping force. The bypass passage is provided with a damping force generating a mechanism for generating relatively small damping force and a damping force control valve for opening and closing the bypass passage.
With the above-described arrangement, when the damping force control valve is opened, the hydraulic fluid in the cylinder is induced to flow mainly through the bypass passage by the sliding movement of the piston caused by the extension and contraction of the piston rod, thereby generating a relatively small damping force. Accordingly, the damping force characteristics are "soft" during both the extension and contraction strokes. When the damping force control valve is closed, the hydraulic fluid in the cylinder is induced to flow through only the main hydraulic fluid passage by the sliding movement of the piston caused by the extension and contraction of the piston rod, thereby generating a relatively large damping force. Accordingly, the damping force characteristics are "hard" during both the extension and contraction strokes. Thus, the damping force characteristics can be changed by opening and closing the damping force control valve.
During the normal running of the vehicle, "soft" damping force characteristics are selected to absorb vibration caused by unevenness of the road surface, thereby improving the ride quality. When the vehicle is turned, accelerated, braked or run at high speed, "hard" damping force characteristics are selected to suppress the change in attitude of the vehicle body, thereby improving the steering stability.
Further, there is a suspension control system in which damping force characteristics are automatically changed in accordance with the road surface conditions, vehicle running conditions, etc., by combining a controller and an actuator with a damping force control type hydraulic shock absorber, thereby improving the ride quality and the steering stability.
In the above-described suspension control system, the appropriate damping force can be quickly obtained for a change in the road surface conditions or vehicle running conditions by making it possible to set a combination of different damping force characteristics for the extension and contraction sides, and the ride quality and the steering stability can be effectively improved.
Accordingly, the present applicant has proposed a damping force control type hydraulic shock absorber in Japanese Patent Application Public Disclosure (KOKAI) No. Hei 5-272570. The proposed hydraulic shock absorber is of the type in which two chambers defined by a piston are communicated with each other by a main hydraulic fluid passage and a bypass passage so that the damping force can be controlled by controlling the passage area of the bypass passage. The proposed hydraulic shock absorber includes a pair of first and second check valves provided in the bypass passage to allow the flow of hydraulic fluid in respective directions which are opposite to each other, a first hydraulic fluid passage that bypasses the first check valve, a second hydraulic fluid passage that bypasses the second check valve, and first and second damping force control valves for controlling the areas of the first and second hydraulic fluid passages, respectively.
According to the above damping force control type hydraulic shock absorber, either the first or second check valve is bypassed by opening one of the first and second damping force control valves and closing the other. Since the bypass passage allows the flow of hydraulic fluid in only one direction, "soft" characteristics are obtained for a piston rod stroke during which the bypass passage allows the flow of hydraulic fluid, while "hard" characteristics are obtained for the other stroke. Thus, it is possible to set a combination of different damping force characteristics for the extension and contraction sides.
The damping force control type hydraulic shock absorber disclosed in Japanese Patent Application Public Disclosure (KOKAI) No. 05-272570 suffers, however, from the following problem. Since one bypass passage is used for both the extension and contraction strokes and the first and second damping force control valves use hydraulic fluid passages in common, it is likely that when the direction of the stroke changes, turbulence will occur in the flow of hydraulic fluid in the damping force control valves, causing noise to be generated.